Pacing at two or more sites using a multipolar lead is conventionally achieved by delivering two or more biphasic pacing pulses in succession. Each pulse consists of a cathodic pacing phase (usually 0.1 to 2 milliseconds (ms) in duration) followed by a second phase, known as the rapid recharge or discharge phase. Rapid recharge consists of an anodic pulse that is usually 4 to 25 ms in duration. The rapid recharge restores the charge that was delivered by the pacing output capacitor during the cathodic output phase. These pulse phases are provided sequentially in order to avoid charge imbalances. That is, if three pulses are delivered, each pulse is typically separated by the duration of the recharge.
FIG. 1 illustrates a set of conventionally delivered biphasic pulses 1, each with 6.5 ms recharge times. In this example, the initial cathodic phase of each pulse is 0.5 ms. The second anodic recharge phase is 6.5 ms, yielding a total pacing and recharge time of about 7 ms per biphasic pulse. In this manner, three pulses are delivered in a period of about 15 ms. One of the limitations of this type of conventional multisite pacing is that simultaneous pacing and recharge is precluded due to the need to provide for recharge after each pulse. That is, the need to provide time for recharge after each cathodic pulse phase limits how close in time the pacing pulses can be packed.
FIG. 2 illustrates a conventional circuit 2 for generating biphasic stimulation pulses. Charge for delivering the stimulation pulse is held in a pacing charge capacitor. A separate charge coupling capacitor blocks direct current to the tip/ring electrodes during pacing and thus avoids electrode corrosion. Assuming the pacing charge capacitor has been properly charged from the voltage source V (e.g. a battery), the delivery of the stimulation pulse consists of two steps: “pacing” and “recharge.” During pacing, a first transistor switch, SWpace, is configured to deliver the cathodic phase of the stimulation pulse, which is of a sufficient voltage amplitude and duration to affect stimulation of the heart (i.e. depolarization and contraction.) More specifically, SWpace is closed to provide a path for charge to flow from the pacing capacitor into the coupling capacitor through the pacing tip and ring electrodes via heart tissue (which is represented by resistance R.) During this cathodic process, the coupling capacitor (typically 5 microfarads) accumulates a small amount of charge, Q=CΔV, subject to a small voltage, ΔV, which is only a fraction of the voltage of supply V. The cathodic phase terminates by opening transistor switch SWpace.
The charge that accumulated on the coupling capacitor during the cathodic phase is then taken off the coupling capacitor during the anodic phase by promptly closing the recharge switch (SWrecharge) for 10 to 25 ms. This anodic phase is also called recharge (or discharge). 10 to 25 ms is usually more than sufficient time to discharge the capacitor through the pacing load, R, which is typically in the range of 500 ohms. The time constant for the recharge is about 2.5 ms. Therefore, 10 to 25 ms is four to ten time constants. Note that a passive recharge resistor is often provided across the SWrecharge switch. The passive recharge resistor has a relatively high resistance of about 40 kilo-ohms to allow for dissipation of any residual charge during a subsequent absolute refractory period. Also, during the absolute refractory period, the charging switch is controlled to recharge the pacing charge capacitor from the voltage source for delivery of another stimulation pulse. Thereafter, the overall process can be repeated to deliver another pulse, which likewise includes both cathodic and anodic phases. Note that the various switches of the circuit are controlled by a microcontroller or other suitable control system (not shown in FIG. 1) of the pacing device. Note also that this is a simplified pacing circuit that only illustrates circuit components pertinent to this discussion. State-of-the-art pacing circuits can include numerous additional components.
FIG. 3 illustrates the voltage shape of a typical biphasic stimulation pulse delivered via the circuit of FIG. 1, including a cathodic pulse/phase 3 and a longer anodic pulse/phase 4. During the initial cathodic phase, SWpace is closed while SWrecharge is open. During the anodic recharge (or discharge) phase, SWrecharge is closed while SWpace open. As noted, typical cathodic stimulation pulse/phases are within the range of 0.1 to 2 ms while the anodic recharge pulse/phases are within the range of 4 to 25 ms, yielding a total pulse duration of typically at least 6 ms up to about to 27 ms. During this period of time, denoted by reference numeral 5, the corresponding sensing channels are blanked or blocked, preventing detection of cardioelectric events such as premature ventricular contractions (PVCs.) Conventionally, each stimulation pulse has this two phase (i.e. biphasic) shape, even when performing multi-site pacing as in FIG. 1.
Thus, FIGS. 2 and 3 illustrate how conventional biphasic stimulation pulses are generated. As already explained, the need to provide a recharge phase after each stimulation pulse limits how closely pacing pulses can be packed when using this type of circuit. To further complicate matters on the electrophysiologic side, if electrodes are within 10 millimeters (mm) of one another, the benefits of multisite pacing in which pulses are separated by in time by a recharge phase will be limited because cardiac conduction to the tissue underlying neighboring electrodes will take place before a second stimulus can be delivered, thus limiting the ability to simultaneously stimulate the electrodes. This phenomenon is illustrated in FIG. 4. A first depolarization propagation diagram 6 shows the propagation of a depolarization triggered by a cathodic pulse from Electrode 2 at time t=0 at 2 ms intervals. Within 10 ms, the depolarization pulse has reached Electrode 1, rendering the tissue at Electrode 1 refractory. Therefore, there would be little or no advantage to delivering a second pacing pulse at Electrode 1 at a time 10 ms after the initial pacing pulse delivered at Electrode 2.
Propagation diagram 7 of FIG. 2 illustrates the advantages of simultaneous delivery of pacing pulses at the two electrodes. Note how effectively the propagation has progressed after 10 ms. This simultaneous pacing may be achieved by simply pacing between the two electrodes, i.e. by pacing using a bipolar configuration rather than a unipolar configuration. Electrode 1 may be used as a cathode and Electrode 2 may be used as an anode. For example, if Electrode 1 stimulates at 0.5 milliamperes of current and Electrode 2 stimulates at 1 milliampere, and if each electrodes is 800 ohms, the total impedance for current driven between the two electrodes is thereby 1600 ohms. Therefore, the voltage threshold for Electrode 1 is 1600 ohms*0.5 milliamperes or 0.8 volts. When the current is increased to 1 milliampere, then both Electrode 1 and Electrode 2 will capture and the common threshold is 1600 ohms*1 milliamperes or 1.6 volts. This provides for simultaneous pacing using one bipolar pacing pulse. In practice, a safety factor (such as 1.7) is typically applied to the magnitude of the pacing stimulus to ensure capture.
Hence, when using only two electrodes in a bipolar pacing configuration, simultaneous delivery of stimulation at two sites is feasible and advantageous. However, this simultaneous pacing technique is not applicable to three or more sites due to the charge balancing issues discussed above.
In an attempt to provide for near simultaneous pacing at three or more sites, two separate output drivers could be used to deliver sequential pacing pulses with the recharge pulses delayed. This is shown in FIG. 5. Within the figure, a set of three pacing pulses are shown, each having a pacing discharge phase 8 followed by a recharge phase 9 that is substantially delayed. In this manner, three stimulation pulses can be delivered nearly simultaneously to three different pacing sites. However, there is a major disadvantage. Sensing is interfered with by the recharge pulses. If the recharge is performed between 5 to 100 ms after the stimulation pulses, the recharge will interfere with the sensing of evoked responses, which is a necessary process when performing capture verification. A later recharge—performed 100 ms or longer after the stimulation pulses—interferes with sensing of PVCs on ventricle or atrial sensing channels. So the pulse packing strategy of FIG. 5 is not considered feasible for use with cardiac sensing/pacing.
Accordingly, it would be highly desirable to provide techniques for providing near simultaneous packed pacing at three or more sites, while providing charge balancing and while also allowing for proper sensing of evoked responses and the like. It is to this end that aspects of the invention are drawn.